The monocarbonate Ca2Al1-xFex(OH)6·½CO3·nH2O AFm system

With the growing emphasis on reducing CO₂ emissions, new hydraulic binders are gaining attention. Understanding the hydration products of these new binders is crucial. This study investigates mixed Al/Fe-CO₃ AFm hydrates. This research proposes two methods to synthesize CO₃-AFm phases with varying proportions of trivalent Al³⁺ and Fe³⁺ cations, followed by a thorough characterization of the samples. Results showed that Al-rich samples crystallized only in the triclinic P1 space group, whereas the Fe-containing CO₃-AFm phase crystallized in both the triclinic P1 and trigonal R$\overline{3}c$ space groups. A complete solid solution between the two end-members (Ca₂Al(OH)₆⋅(CO₃)_0.5⋅2.5H₂O and Ca₂Fe(OH)₆⋅(CO₃)_0.5⋅2.5H₂O) was confirmed with triclinic symmetry, whereas a partial solid solution was observed for the rhombohedral Ca₂Al_1-xFe_x(OH)₆⋅(CO₃)_0.5⋅3H₂O higher hydrate with ∼0.33 ≤ x ≤ 1. This study enhances the characterization of Al/Fe-mixed CO₃-AFm phases that develop during the hydration of binders containing iron and aluminum, offering crucial insights for developing new sustainable construction materials.